In recent years, attention has been drawn to multicarrier communication apparatuses using an OFDM (Orthogonal Frequency Division Multiplexing) system as apparatuses enabling high-rate radio transmission, because such communication apparatuses have resistance to multipath and fading and permit high-quality communication. Further, using modulation diversity techniques has been proposed for performing phase rotation and interleaving on modulation symbols such as QPSK (Quadrature Phase Shift Keying) and thereby enabling the diversity effect to be obtained.
Modulation diversity is described in Non-patent Document 1, for example. Referring to FIG. 1, modulation diversity will be described briefly. FIG. 1 shows a case of using QPSK (Quadrature Phase Shift Keying) as a modulation scheme as an example. First, a transmitting side rotates a phase of a symbol mapped on the IQ plane by a predetermined angle. Next, the transmitting side performs interleaving on an I (in-phase) component and Q (quadrature) component using uniform or random interleavers respectively for the I component and Q component. By this means, signals subjected to inverse fast Fourier transform (IFFT) are processed such that the I component and Q component of the symbol prior to interleaving are mapped to different subcarriers. In FIG. 1, the I component is mapped to a subcarrier B, while the Q component is mapped to a subcarrier A.
First, a receiving side performs fast Fourier transform (FFT), and thereby extracts the I component and Q component multiplexed on the subcarriers. Next, the receiving side performs deinterleaving, and thereby restores the I component and Q component to original arrangements. Then, the receiving side performs demapping processing based on a constellation of the restored I component and Q component, and thereby obtains reception data.
Here, assuming that the subcarrier A has a good channel state and that the subcarrier B has a poor channel state, the receiving side obtains a constellation distorted in the Q-component direction as shown in FIG. 1. By this means, it is possible to maintain a signal point distance on the constellation at a relatively long, and to restore bits in a packet accurately averagely at a demapping. Thus, in modulation diversity, even when the fading variation occurs on each subcarrier due to multipath fading, the same effect can be obtained as in dispersing a SNR (Signal-to-Noise Ratio) in the subcarrier direction to make a correction. As a result, the modulation symbol undergoes the variation as if the signal is transmitted on an AWGN (Additive White Gaussian Noise) communication path, and the diversity gain can thus be obtained.
FIG. 2 illustrates a configuration of multicarrier transmission apparatus 10 that performs modulation diversity transmission processing. FIG. 3 illustrates a configuration of multicarrier reception apparatus 30 that receives and demodulates signals from the apparatus 10.
Multicarrier transmission apparatus 10 has modulation diversity modulation section 11, and inputs transmission data to mapping section 12 in modulation diversity modulation section 11. Mapping section 12 maps the transmission data on symbols on the IQ plane corresponding to a modulation scheme such as BPSK, QPSK, 16QAM and the like.
Phase rotation section 13 rotates the phase of a mapped symbol by a predetermined angle. IQ separating section 14 separates the symbol with the phase rotated into the I component and Q component. The separated I and Q components are temporarily stored respectively in buffers 15 and 16. The Q component stored in buffer 16 is interleaved in interleaver 17 and output to combining section 18. In addition, although FIG. 2 illustrates the case of interleaving the Q component, the I component may be subjected to interleaving processing, or both the I and Q components may be subjected to interleaving processing.
Combining section 18 combines the I component output from buffer 15 and the Q component output from interleaver 17 to place back in a constellation. A modulation diversity symbol is thereby obtained. The modulation diversity symbol is multiplexed on a predetermined subcarrier in serial/parallel transform (S/P) section 19 and inverse fast Fourier transform (IFFT) section 20. In other words, serial/parallel transform (S/P) section 19 and inverse fast Fourier transform (IFFT) section 20 map the modulation diversity symbol to any one of a plurality of subcarriers orthogonal to one another, and sequentially modulate each of the subcarrier with the modulation diversity symbol.
Thus, in multicarrier transmission apparatus 10, since interleaver 17 interleaves the Q component, the I component is fixed to some subcarrier, while a subcarrier to which the Q component is mapped varies according to interleaving patterns. An IFFT-processed signal is subjected to radio transmission processing such as analog/digital conversion processing, upconverting and the like in radio transmission section 21, and then transmitted via antenna 22.
Multicarrier reception apparatus 30 that receives and demodulates signals transmitted from multicarrier transmission apparatus 10 has modulation diversity demodulation section 31. In multicarrier reception apparatus 30, radio reception section 33 performs radio reception processing such as downconverting, analog/digital conversion processing and the like on a radio signal received in antenna 32 to output to fast Fourier transform (FFT) section 34. FFT section 34 extracts a modulation diversity symbol multiplexed on each subcarrier. Phase compensation section 35 compensates the extracted modulation diversity symbol for a phase variation occurring during propagation. The phase-compensated modulation diversity symbol is output to IQ separating section 36 inmodulation diversity demodulation section 31.
IQ separating section 36 separates symbols into the I component and Q component. Of the separated components, IQ separating section 36 outputs one component that is not interleaved at the transmitting side to combining section 40 via buffer 37 without any processing, while outputting the other component interleaved at the transmitting side to deinterleaver 39 via buffer 38. Deinterleaver 39 performs processing inverse to that in interleaver 17, and thereby restores interleaved components to an original arrangement and outputs to combining section 40. As a result, combining section 40 obtains a symbol comprised of the original pair of I component and Q component.
Phase rotation section 41 rotates the phase of the combined symbol in the inverse direction by the same angle to/as in phase rotation section 13 of the transmitting side. Demapping section 42 demaps the phase-rotated symbol and thereby outputs reception data.
Here, FIG. 4 illustrates modulation symbols that are subjected to QPSK modulation in mapping section 12 and then phase rotation processing of 26.6° in phase rotation section 13. As can be seen from FIG. 4, the modulation symbols are mapped at points of 16QAM at an angle of 26.6 degrees.
FIG. 5 illustrates I components and Q components combined in combining section 18. In FIG. 5, numerals “1” to “4” denote respective numbers of four QPSK symbols. I components are not interleaved, and therefore, the I components of modulation symbols are input to combining section 18 in the same order. In contrast thereto, the order of the Q components is rearranged by interleaving and input to combining section 18.
Here, four modulated symbols in mapping section 12 are expressed as S0=[S10 S20 S30 S40]=[(1 1), (−1 1), (1 −1), (−1−1)], where numerical subscripts “1” to “4” respectively represent four symbols obtained by QPSK, and a numerical superscript “0” represents a transmission symbol. Then, for example, using the I component and Q component, symbol 1 is represented as S10=(S1I0, S1Q0).
When Q components are interleaved with an interleaving pattern as shown in FIG. 5, symbol S obtained in combining section 18 is represented as S=[(S1I0, S4Q0), (S2I0, S1Q0), (S3I0, S2Q0), (S4I0, S3Q0)]=[(1 1−1−1), (−1 1 1 1), (1 −1 −1 1), (−1−1 1−1)]. This corresponds to transmitting either point on 16QAM corresponding to the interleaving pattern.
Assuming that the interleaving pattern as shown in FIG. 5 is used at the transmitting side, since an original first symbol is transmitted in the received first symbol and second symbol, to obtain the original first symbol, the receiving side separates the received symbols into I components and Q components, deinterleaves the Q components, and obtains the original first symbol by combining. Here, FIG. 6 shows a constellation in the case of obtaining an original one symbol by combining when a received symbol is represented as Sr1=[S1r1, S2r1, S3r1, S4r1] (where numerical subscripts “1” to “4” respectively represent different symbols, and a numerical superscript “r1” represents a received symbol.) Four points in FIG. 6 are candidates for reception points. In addition, although in FIG. 6, length of |S1Ir1| and |S2Qr1| are shown with almost the same, the lengths are actually different from each other due to the difference in fading and the like imposed on the symbol and four points in the figure form a parallelogram.
Thus, it is a feature of the modulation diversity system to transmit components of an original symbol in different symbols and to avoid the both components of symbol restored at the receiving side becoming smaller. Particularly, when this system is used in OFDM, it is possible to obtain large diversity gains because each subcarrier undergoes different fading.
[Non-patent Document 1] Signal space diversity: a power- and bandwidth-efficient diversity technique for the Rayleigh fading channel, Boutros, J.; Viterbo, E.; Information Theory, IEEE Transactions on Volume: 44 Issue: 4, July 1998, Page(s) z : 1453 -1467